1. Field of the Invention
The present invention relates to a display device, and more particularly to a display device including a backlight unit having positioning ribs formed on the luminescent surface thereof and a panel unit mounted on the luminescent surface of the backlight unit wherein the position of the panel unit is defined by the positioning ribs.
2. Description of the Related Art
In general, a liquid-crystal display (LCD) device has a structure configured by sandwiching a liquid-crystal (LC) panel unit between a backlight unit and a shield front. FIG. 17 is a perspective view showing the configuration of a conventional LCD device as observed from the front side. FIG. 18 is an exploded perspective view of the LCD device of FIG. 17. The LCD device 100 includes a backlight unit 11 having a rectangular luminescent surface 11a on the front side, a LC panel unit 12 mounted on the luminescent surface 11a and having a light-switching function, and a shield front 13 defining a display surface 13a for the panel unit 12.
The LC panel unit 12 includes a display panel 21, a plurality of TCPs (Tape Carrier Packages) 22 coupled to the display panel 21, and a circuit board 23 coupled mechanically and electrically to the TCPs 22. ICs for driving the display panel 21 are mounted on the TCPs 22, and the circuit board 23 outputs control signals controlling the drive of the display panel 21 by the TCPs 22.
The display panel 21 includes a LC layer (not shown), front and rear transparent substrates 24, 25 bonded together at a bonding surface to sandwich therebetween the liquid crystal layer, and a pair of polarizing plates (not shown) disposed on the external surface of the transparent substrates 24, 25. The display panel 21 has thereon a plurality of pixels defined in a two-dimensional array. The rear substrate 24 is configured as an active matrix substrate, mounting thereon switching elements such as TFTs (Thin Film Transistors) and the like disposed in a matrix for controlling the switching of the pixels. The rear substrate 24 is thus hereinafter referred to as TFT substrate 24. The front substrate 25 mounts thereon color filters (CFs) having a color which corresponds to each pixel, and is hereinafter referred to as a counter substrate or CF substrate 25. By driving the TFT elements via the circuit board 23 and the TCPs 22, the display panel is controlled for each pixel.
The TCPs 22 and the circuit board 23 are disposed, for example, on the periphery of the display panel 21 along two adjacent sides of the rectangular display panel 21. On these two sides, the edge of the TFT substrate 24 protrudes from the edge of the counter substrate 25 as observed normal to the display panel 21, and the TCPs 22 are coupled to the peripheral area of the TFT substrate 24 exposed from the edge of the counter substrate 25.
The backlight unit 11 is provided with, on the front side thereof, a frame-like backlight chassis 31 defining the luminescent surface 11a of the backlight unit 11. On the backlight chassis 31, a positioning rib set including a plurality of positioning ribs 42 is formed in the vicinity of four corners of the luminescent surface 11a for defining the position of the display panel 21. A pair of positioning ribs 42 are formed on each corner of the display panel 21, with the corner being interposed between the pair of positioning ribs 42. Each rib has a shape of elongated cuboid and is disposed along an edge of the display panel 21. Further, in consideration of the range of variation of the peripheral dimensions of the display panel 21 and the range of variation of the distance between the positioning ribs 42, each rib is disposed so as to form a clearance of 0.2 to 0.3 mm between the edge of the display panel 21 and the positioning ribs 42.
FIG. 19A is a top plan view showing the LCD device of FIG. 18 in which the LC panel unit 12 is mounted on the backlight unit 11, and FIG. 19B is a sectional view showing a section taken along line b-b of FIG. 19A. The backlight unit 11 is, for example, an edge-emission type backlight unit, and consecutively includes, on the luminescent surface 11a thereof, an optical sheet 33, a optical guide plate 34 and a reflection sheet (not shown). The backlight unit 11 includes a lamp or light source (not shown) adjacent to both the side edges of the optical guide plate 34. Around the luminescent surface 11a, the backlight chassis 31 has a flat mounting surface 35 on which the TFT substrate 24 is mounted, and a recess 36 formed nearer to the luminescent surface 11a than the mounting surface 35 and being capable of accommodating a polarizing plate 26. Numeral 28 indicates the edge of the display area of the display panel 21.
In the assembly process of the LCD device 100, the four corners of the display panel 21 are positioned so as to be located inside the positioning ribs 42 in the in-plane direction of the display panel 21, and then, the LC panel unit 12 is mounted on the backlight unit 11. In this process, the TFT substrate 24 is mounted on the mounting surface 35 with polarizing plate 26 being received in the recess 36 of the backlight chassis. Further, the shield front 13 is overlaid thereon so as to cover the backlight unit 11 and the LC panel unit 12, and is pressed down to a predetermined position, thereby completing the assembly. The backlight unit 11 and the shield front 13 are fixed to each other by means of screwing, caulking, pawl fitting or the like.
It is known that the conventional LCD device 100 incurs the problem of poor image quality in which shadow of dust is projected onto the display surface 13a due to the dust adhered onto the luminescent surface 11a of the backlight unit. This poor image quality is attributable to the following causes. In the LCD device 100, the TFT substrate 24 and the counter substrate 25 is configured by a transparent body such as made of glass, and the edge of the transparent body is not polished, and has minute irregularities such as shown in FIG. 19B by reference numeral 101. Accordingly, if the LCD device 100 is subjected to vibration, the edge of the transparent substrate body, which is in contact with the positioning ribs 42, suffers from abrasion with respect to the same to generate the dust. This dust passes through the gap between the bottom surface of the TFT substrate 24 and the mounting surface 35 of the backlight unit 11, penetrates into the lower part of the display panel 21 and adheres onto the luminescent surface 11a of the backlight unit.
In order to solve the above-described problem, measures of forming an inclined surface on the side of each of the positioning ribs 42 opposing the display panel 21 is employed. FIG. 20 is a sectional view showing the LCD device which has an inclined surface similar to the surface of equivalent to FIG. 19B. In this LCD device 100, the inclined surface 112 is formed on the side of the display panel 21 opposing the positioning ribs 42.
In the LCD device 100, the height of the boundary between a positioning wall 111 and the inclined surface 112 opposing the edge surface of the display panel 21 is set smaller than the height of the top surface of the TFT substrate 24. As a result thereof, when the edge portion of the display panel 21 is brought into contact with the positioning ribs 42, only a part of the edges of the TFT substrate 24 is in contact with the positioning ribs 42. It is considered to be able to suppress the generation of dust due to friction between the display panel 21 and the positioning ribs 42 by reducing the contact area between the edge of the display panel 21 and the positioning ribs 42. The LCD device in which the inclined surface is formed on the side of the positioning ribs is described, for example, in JP-2001-4983A.
Research was conducted by the inventor for investigating the incidence rate of the poor image quality after actually manufacturing the LCD device 110 of FIG. 20, and it was found in the research that the poor image quality due to the dust cannot be suppressed satisfactorily. The reason why the poor image quality due to the dust cannot be suppressed satisfactorily is conceived as follows.
The display panel 21 is formed by bonding the TFT substrate 24 and the counter substrate 25 together, with a LC layer sandwiched therebetween. Accordingly, depending on processing accuracy of the bonding, the position of the edge of the TFT substrate 24 and the position of the edge of the counter substrate 25 cannot be aligned perfectly, thereby leaving a certain degree of misalignment therebetween. Depending on the bonding surface between the TFT substrate 24 and the counter substrate 25, the edge of the counter substrate 25 may protrude outwards from the edge of the TFT substrate 24.
In the LCD device shown in FIG. 20, the TFT substrate 24 and the counter substrate 25 have a thickness of, for example, 0.7 mm, and in consideration of the range of variation in the dimension of the positioning ribs 42, the boundary between the positioning wall 111 and the inclined surface 112 is set at a height of about 0.5 to 0.6 mm from the mounting surface 35 of the backlight unit so that a part thereof does not contact with the counter substrate 25. The angle of the inclined surface 112 is set, for example, at about 10 to 30° so as to be able to mount the LC panel unit 12 with ease during the assembly of the LCD device 110.
The misalignment between the edge of the TFT substrate 24 and the edge of the counter substrate 25 amounts to, for example, a maximum of about 0.4 mm. Accordingly, in case of largely protruded edge of the counter substrate 25, even if the inclined angle is set at 30°, as shown by the numeral 113 of FIG. 21, the corner of the counter substrate 25 abuts the inclined surface 112 of the positioning ribs 42 before the TFT substrate 24 is brought into contact with the positioning ribs 42. The dust causing the above-described poor image quality is generated by friction between the corner of the counter substrate 25 and the inclined surface 112 of the positioning ribs.
Here, countermeasures can be conceived that the angle of the inclined surface 112 is increased sufficiently for preventing the corner of the counter substrate 25 from contacting with the inclined surface 112 of the positioning ribs. However, according to the simulation of the present inventor, assuming that the misalignment between the edge of the TFT substrate 24 and the edge of the counter substrate 25 amounts to a maximum of 0.4 mm, even if the height of the boundary between the positioning wall 111 and the inclined surface 112 is reduced to a height of 0.5 mm from the mounting surface 35 of the backlight unit, contact of the corner of the counter substrate 25 with the inclined surface 112 of the positioning ribs cannot be prevented unless the inclined angle is set to larger than 68°, as shown in FIG. 22A.
However, if the angle of the inclined surface 112 is excessively large, as shown in FIG. 22B, the LC panel unit 12 is likely to be stopped after overriding the positioning ribs 42 during the assembly of the LCD device 120. Accordingly, if the constituent members such as the shield front 13 are attached onto the LCD device 12 in this situation, there may occur a dimensional error, or the LC panel unit 12 may be damaged due to stresses. In particular, in a LCD device having a display panel 21 of large size, since the LC panel unit 12 is heavy, the LC panel unit 12 is more likely to be stopped on the ribs 42.